Overwintering Dahlias: How to Store Tubers and Protect Them Through Winter

Dahlias are tender perennials. The tubers survive from year to year and multiply underground each season, but a hard frost will kill them. In their native Mexico, this is not a problem. In the UK, it means you need to do something about winter. The question is what.

There are two approaches. You can lift the tubers and store them indoors. Or you can leave them in the ground under a thick mulch. We do both, depending on where in the garden the dahlias are planted, and we have been doing it for years. Both work. Neither is difficult. This guide covers both methods in detail.

	Pros	Cons
Leave Tubers In the Ground	No work, just apply mulch, and possibly slug protection.	Tubers might rot or be eaten by slugs. Flowering season will be delayed and shorter.
Lift and Store Tubers	Tubers are safe from harm. Start them in pots early the following year for the best flowering season. Divide large clumps.	Another chore! Needs storage space.

For general growing advice, see our main how to grow dahlias guide. For container-grown dahlias, which have their own overwintering considerations, see growing dahlias in pots.

When to Lift Dahlia Tubers

Wait for the first proper frost to blacken the foliage. Not a light touch of frost that nips the top leaves. A proper frost that turns the whole plant to mush overnight. This usually happens sometime in late October or November, depending on where you are and what kind of autumn we are having.

Once the frost has done its work, cut the stems down to about 10cm above ground level. Leave the short stumps as handles. Then act promptly. Don't leave the cut stems sitting in the ground for weeks. Water runs down the hollow stems and into the crown, and that, dear gardener, is how tubers rot before you get round to lifting them.

If you live in the south or south-west and your first hard frost doesn't arrive until late November or even December, you can leave the dahlias growing and flowering as long as they are happy. We have had dahlias in flower in November some years. When frost finally comes, cut and lift within a few days.

How to Lift Dahlia Tubers

Lifting dahlia tubers is not complicated, but you can damage them by being careless. The tubers will have multiplied over summer and the clump is bigger than what you planted. Dig wide. A garden fork is better than a spade because it is less likely to slice through a tuber.

Push the fork in about 30cm out from the stem stump, going straight down. Lever gently. Work your way round the clump, loosening on all sides. Then lift the whole thing out. Don't pull by the stem stumps. They snap off and you lose the crown, which is where next year's growth buds are.

Shake off as much soil as you can. If the soil is heavy clay and clinging, leave the clump somewhere sheltered for an hour or two until the soil dries enough to knock off. You don't need to wash the tubers, and in fact it is better not to. A thin coating of dry soil protects the skin.

Turn the clump upside down and leave it for a day or two on newspaper in a shed or garage. This lets water drain out of the hollow stems. Water trapped inside the stems is the most common cause of rotting in storage.

How to Store Dahlia Tubers Over Winter

The ideal storage conditions for dahlia tubers are cool, dark, and frost-free. Between 5°C and 10°C is perfect. Too warm and the tubers dry out and shrivel. Too cold and they freeze. Too damp and they rot. An unheated room, a frost-free garage, a shed insulated enough to keep above freezing, or a cellar all work well.

Pack the tubers in something that breathes. Dry compost, dry wood shavings, dry vermiculite, or even scrunched-up newspaper. The packing material absorbs any surface moisture and stops the tubers touching each other, which is where rot spreads. Cardboard boxes or paper sacks are fine. Plastic bags are not, because they trap moisture.

Label everything. You will not remember which tuber is which in March. Write the variety name on a piece of masking tape stuck to the stem stump, or on the box. Trust me on this.

Check the tubers once a month over winter. Pick each one up and feel it. A healthy tuber feels firm, like a potato. If one has gone soft or mushy, throw it away before the rot spreads to its neighbours. If a tuber is shrivelling and feels very light, it is drying out. Mist it lightly with water or pack it more tightly.

Don't cut the tubers apart until spring. Dividing now creates fresh wounds that invite infection over winter. Wait until March when the eyes are swelling and you can see exactly where to cut.

Leaving Dahlias in the Ground Over Winter

This is what we do with most of ours, and it works more often than the books suggest. The key is a thick mulch. Really thick. At least 15cm of dry material piled over the cut stems and covering an area wider than the tuber clump below.

Once frost has killed the top growth and you have cut the stems to about 10cm, pile on the mulch. Dry straw is excellent. Dry leaves work well. Bark chips are fine. The material needs to be dry when you apply it. Wet mulch holds moisture against the crown and that defeats the purpose.

If your soil is free-draining, sandy, or sits on chalk, leaving dahlias in the ground works well. The tubers may get cold but they are unlikely to sit in wet soil, which is the real killer. If your soil is heavy clay that puddles after rain, lifting is safer. It's the combination of cold and wet that rots tubers, not cold alone.

In the south of England, parts of the south-west, and sheltered urban gardens where winter temperatures rarely drop below minus 5°C for more than a night or two, mulching in place works reliably. In the midlands and north, colder inland valleys, and exposed sites, the risk is higher. We would still try it, but with varieties we could afford to lose rather than expensive or irreplaceable ones.

Remove the mulch in early spring, around late March or early April, before the new shoots have to fight their way through a foot of wet straw. By then you should see the first swelling eyes on the crown. If the tuber feels firm, it has survived. If it is soft and mushy, it hasn't.

Can I Leave Potted Dahlias in Their Pots Over Winter?

Yes. If your dahlias are growing in pots, you don't need to lift the tubers at all. Wait until frost blackens the foliage (or mid-November if they're in a frost-free spot and still green), cut the stems to about 10cm, and move the whole pot somewhere frost-free for winter. An unheated greenhouse, a shed, a garage. Don't water them. The compost should be barely damp, not wet.

In spring, bring the pots back out after the last frost and start watering gently. The tubers will reshoot. If the clump has outgrown its pot, tip it out and divide before repotting in fresh compost. More on container growing in our dahlias in pots guide.

Dividing Dahlia Tubers in Spring

Whether you lifted your tubers in autumn or left them in the ground, spring is the time to divide them if the clump has grown large. A dahlia tuber that spent one season in the ground can easily double or triple in size, and a congested clump produces more foliage than flowers.

Wait until March when the eyes begin to swell. You need to see where the growing points are before you cut. Each division must have at least one eye and a section of crown attached to the tuber body. Use a clean, sharp knife. Cut cleanly. Let the cut surface dry for a day before planting or potting up.

If you are not sure what an eye looks like or where the crown is, read our tuber anatomy guide. It makes the difference between a confident cut and a nervous guess.

A single tuber bought this spring can realistically become four or five separate plants by the following year. That is very good value. It also means you can give spare tubers to friends, which is one of the nicest things about growing dahlias.

Frequently Asked Questions

Can I leave dahlias in the ground over winter in the UK?

Yes, in many parts of the UK, provided you mulch heavily. Cover the cut stems with at least 15cm of dry straw, leaves, or bark after the first hard frost. Free-draining soils and sheltered gardens in the south are the safest. Heavy clay in exposed northern sites is the riskiest. We leave most of ours in the ground under mulch and lose very few.

How do I know if my stored dahlia tubers are still alive?

Feel them. A healthy tuber feels firm, like a potato. Soft, mushy tubers have rotted. Shrivelled, feather-light tubers have dried out. Check monthly over winter and discard any that have gone bad before the rot spreads. In spring, look for swelling eyes on the crown, which confirm the tuber is alive and ready to grow.

When should I bring stored dahlia tubers out of storage?

Late March to early April. Bring them into the light and warmth to encourage the eyes to swell. You can pot them up in small pots of damp compost to start them growing before planting out after the last frost. Full method in our starting tubers in spring guide.

What is the best temperature to store dahlia tubers?

Between 5°C and 10°C. A frost-free garage, shed, or unheated room is ideal. Too warm and the tubers dry out. Below freezing and they die. A maximum-minimum thermometer in your storage area takes the guesswork out of it.

Do I need to divide dahlia tubers every year?

Not every year, but every two or three years is a good idea. A dahlia clump that gets too big produces masses of foliage and fewer flowers. Dividing in spring gives you fresh, vigorous plants and spare tubers to give away. Wait until the eyes are visible before you cut. See our tuber anatomy guide for help identifying where to cut.

Browse our full range of dahlia tubers. We stock over 80 varieties, all supplied as A-grade Dutch-grown tubers that we double-check before dispatch. Plant them this spring, look after them over winter, and they will come back bigger and better next year.